The invention relates generally to x-ray lithography and more particularly to soft x-ray projection lithography, and x-ray optical devices for performing same.
After its initial demonstration in 1972, proximity print x-ray lithography (PPXRL) appeared to be the lithography of choice for future submicron work. PPXRL uses "hard" x-rays (wavelengths of 0.3 to 2 nm) to expose a mask consisting of an x-ray absorber pattern (usually gold or tungsten) on an x-ray transparent membrane (silicon, silicon nitride, boron nitride, etc) at some finite distance (5 to 50 microns) from a resist coated wafer. Unfortunately, PPXRL has several fundamental constraints arising from diffraction effects, penumbra and secondary photoelectron range which may limit replications to linewidths greater than 200 nm. Even with these limitations, it appeared that PPXRL would be the primary lithographic tool for linewidths from 200 nm to 1 micron and would meet lithographic needs for many years to come.
However, PPXRL has not reached expectations. There are three primary reasons for this: (1) a high brightness x-ray source was needed to obtain high wafer throughput, (2) the hard x-rays required masks with thick absorber patterns and high-aspect-ratio submicron structures which are difficult to produce and (3) the thin, x-ray transparent membranes have had severe distortion and lifetime problems. While solutions to these problems were pursued, optical lithography has advanced its capabilities so that it can now replicate 500 nm linewidths. This has reduced the immediate need for PPXRL. With the fundamental resolution limitations of PPXRL and some mask issues still unresolved, it is questionable if PPXRL will ever meet original expectations.
New advances in the field of x-ray optics have been responsible for many new x-ray optical components such as normal incidence soft x-ray mirrors, beamsplitters and highly dispersive multilayer mirrors. These new optical components have made it possible to design and build new instruments such as x-ray microscopes, telescopes, waveguides and interferometers. It is highly desirable to apply these new x-ray optical components to produce a soft x-ray projection lithography (XRPL) system which is capable of projecting a magnified or demagnified image of an existing pattern from a mask onto a resist coated substrate.